When a ship moves through the water the drag resistance or water frictional forces which must be overcome are responsible for as much as half of the power consumed in operation of the vessel. The surface condition of the hull is a major factor inducing drag. It is therefore desirable to have an extremely smooth surface on the hull and paint formulations have been developed that are very smooth when cured and/or are polished by moving water to provide an extremely smooth surface. It is desirable to have a coating material that exhibits this polishing action to produce a microsmooth surface to minimize the drag penalty due to microroughness.
Fouling of the hull by pestiferous marine organisms is a major source of drag. The use of antifouling protective coating on a ship's hull is a primary approach to controlling fouling and the resulting drag. The antifouling coating inhibits growth of marine organisms on the hull to keep it smooth. Coatings can also be used on static structures exposed to seawater to minimize growth of organisms that could cause deterioration of such structures.
A truly effective antifouling coating meets at least three criteria: (1) It will possess broad spectrum antifouling efficacy (i.e., inhibit growth of a broad variety of organisms) for extended periods of time, usually three years; (2) it will possess a smooth surface so as not to cause a microroughness drag penalty; and (3) it will actively reduce drag by reducing the roughness profile of the surface.
To meet the first criterion it is necessary to deliver to the surface of the coating in a controlled fashion, minimum effective amounts of toxin or fouling control agents. The amount of toxin delivered at the surface should not be substantially above the minimum effective amount for inhibiting fouling to avoid premature depletion of the antifouling agent.
One technique for controlling release of toxin involves the use of latent toxicants which are activated by an environmental or chemical trigger such as hydrolysis. This is the principal behind the operation of organotin polysiloxane materials as described in U.S. Pat. No. 4,080,190. In these materials a trisubstituted organotin moiety is chemically bonded to a macromolecular polysiloxane backbone. Through hydrolysis the organotin moiety is gradually liberated and diffuses to the surface of the coating as the active fouling control agent.
The organotin polysiloxane materials can act as binders, co-resins, or toxic pigments or additives depending on the tin to silicon ratio and related physical form. A low tin to silicon ratio permits the organotin polysiloxane to perform as a binder. Such material is primarily inorganic in nature though the presence of the organotin groups do impart a certain degree of organic character. This enhances compatability with organic materials and better adhesion to metal substrates, for example, than a polysiloxane without organotin substitution.
As a binder the organotin polysiloxane can serve as a matrix for essentially inorganic fillers and pigments. The coating is microporous allowing continual release of the toxic agent; that is, an organotin radical is formed in situ through hydrolysis of the tin-oxygen-silicon bond. Such continual release of toxicant avoids surface passivation as frequently occurs in conventional copper based antifouling coatings. Since this type of formulation is microporous, performance is essentially independent of turbulence; that is, sufficient toxicant is leached to the surface for preventing fouling under either static or dynamic conditions.
With a higher tin to silicon ratio the organotin polysiloxane can be an additive in a coating composition using a variety of binders. In such a composition the release of toxicant is a function of the properties of the binder plus hydrolysis characteristics of the organotin polysiloxane.